[TAPESTRY-806] (T3 only) Multithreading problems possibly due to double-checked locking

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.5
• Fix Version/s: 3.0.5
• Component/s: Framework
• Environment: Unspecified, except for "8-CPU server"
• Assignee: Jesse Kuhnert
• Reporter: Nick Westgate
• Created: 15/Dec/05 13:54
• Updated: 30/Apr/06 16:53
• Resolved: 14/Mar/06 21:34

Description

Flawed synchronization code using "double-checked locking" is used in these methods:

(1) org.apache.tapestry.enhance.DefaultComponentClassEnhancer.getEnhancedClass()
(2) org.apache.tapestry.enhance.javassist.EnhancedClassFactory.getObjectType()

Suggested fix for (1) is:

public Class getEnhancedClass(IComponentSpecification specification,
String className)
{
synchronized (specification)
{
Class result = getCachedClass(specification);

if (result == null)
{
result = getCachedClass(specification);
if (result == null)

{ result = constructComponentClass(specification, className); storeCachedClass(specification, result); }

}
}

return result;
}

Original bug report, from Michael Prescott ( [e-mail] ) in users list is:

We're coming across an unusual exception that occurs when our
application is under load. Below is an example stack trace, although
the problem can occur while enhancing any of a number of things, so it
doesn't seem specific to one part of our application. The root
exception is always the same, however:

javassist.CannotCompileException: duplicate method: <init>

Since it seemed to be related to concurrency, we were reading through
the Tapestry classes along this stack, and found what looks like a
threading problem in DefaultComponentClassEnhancer.java:

public Class getEnhancedClass(IComponentSpecification specification,
String className)
{
Class result = getCachedClass(specification);

if (result == null)
{
synchronized (this)
{
result = getCachedClass(specification);
if (result == null)

{ result = constructComponentClass(specification, className); storeCachedClass(specification, result); }

}
}

return result;
}

This is double-checked locking, which isn't safe given instruction
reordering and thread-specific copies of variables, and all that
multi-CPU magic. This problem is occurring on an 8-CPU server, which
I'm guessing is enough processors to expose the subtle ways in which
double-checked locking can fail.

Does this ring any bells for anyone? I realize this is a somewhat
nebulous query.

Michael

org.apache.tapestry.ApplicationRuntimeException: A code generation error
occured while enhancing class org.apache.tapestry.link.GenericLink.
at
org.apache.tapestry.enhance.DefaultComponentClassEnhancer.constructCompo
nentClass(DefaultComponentClassEnhancer.java:151)
at
org.apache.tapestry.enhance.DefaultComponentClassEnhancer.getEnhancedCla
ss(DefaultComponentClassEnhancer.java:97)
at
org.apache.tapestry.pageload.PageLoader.instantiateComponent(PageLoader.
java:603)
at
org.apache.tapestry.pageload.PageLoader.createImplicitComponent(PageLoad
er.java:569)
at
org.apache.tapestry.BaseComponentTemplateLoader.createImplicitComponent(
BaseComponentTemplateLoader.java:295)
at
org.apache.tapestry.BaseComponentTemplateLoader.process(BaseComponentTem
plateLoader.java:237)
at
org.apache.tapestry.BaseComponentTemplateLoader.process(BaseComponentTem
plateLoader.java:172)
at
org.apache.tapestry.BaseComponent.readTemplate(BaseComponent.java:100)
at
org.apache.tapestry.BaseComponent.finishLoad(BaseComponent.java:135)
at
org.apache.tapestry.pageload.PageLoader.constructComponent(PageLoader.ja
va:519)
at
org.apache.tapestry.pageload.PageLoader.createImplicitComponent(PageLoad
er.java:576)
at
org.apache.tapestry.BaseComponentTemplateLoader.createImplicitComponent(
BaseComponentTemplateLoader.java:295)
at
org.apache.tapestry.BaseComponentTemplateLoader.process(BaseComponentTem
plateLoader.java:237)
at
org.apache.tapestry.BaseComponentTemplateLoader.process(BaseComponentTem
plateLoader.java:172)
at
org.apache.tapestry.BaseComponent.readTemplate(BaseComponent.java:100)
at
org.apache.tapestry.BaseComponent.finishLoad(BaseComponent.java:135)
at
org.apache.tapestry.pageload.PageLoader.constructComponent(PageLoader.ja
va:519)
at
org.apache.tapestry.pageload.PageLoader.createImplicitComponent(PageLoad
er.java:576)
at
org.apache.tapestry.BaseComponentTemplateLoader.createImplicitComponent(
BaseComponentTemplateLoader.java:295)
at
org.apache.tapestry.BaseComponentTemplateLoader.process(BaseComponentTem
plateLoader.java:237)
at
org.apache.tapestry.BaseComponentTemplateLoader.process(BaseComponentTem
plateLoader.java:172)
at
org.apache.tapestry.BaseComponent.readTemplate(BaseComponent.java:100)
at
org.apache.tapestry.BaseComponent.finishLoad(BaseComponent.java:135)
at
org.apache.tapestry.pageload.PageLoader.constructComponent(PageLoader.ja
va:519)
at
org.apache.tapestry.pageload.PageLoader.createImplicitComponent(PageLoad
er.java:576)
at
org.apache.tapestry.BaseComponentTemplateLoader.createImplicitComponent(
BaseComponentTemplateLoader.java:295)
at
org.apache.tapestry.BaseComponentTemplateLoader.process(BaseComponentTem
plateLoader.java:237)
at
org.apache.tapestry.BaseComponentTemplateLoader.process(BaseComponentTem
plateLoader.java:172)
at
org.apache.tapestry.BaseComponent.readTemplate(BaseComponent.java:100)
at
org.apache.tapestry.BaseComponent.finishLoad(BaseComponent.java:135)
at
org.apache.tapestry.pageload.PageLoader.constructComponent(PageLoader.ja
va:519)
at
org.apache.tapestry.pageload.PageLoader.loadPage(PageLoader.java:764)
at
org.apache.tapestry.pageload.PageSource.getPage(PageSource.java:152)
at
org.apache.tapestry.engine.RequestCycle.getPage(RequestCycle.java:195)
at
org.apache.tapestry.engine.ActionService.service(ActionService.java:125)
at
org.apache.tapestry.engine.AbstractEngine.service(AbstractEngine.java:88
9)
at
org.apache.tapestry.ApplicationServlet.doService(ApplicationServlet.java
:198)
at
net.exchangesolutions.sn.ui.tapestry.SynchronizedServlet.doService(Synch
ronizedServlet.java:37)
at
org.apache.tapestry.ApplicationServlet.doGet(ApplicationServlet.java:159
)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:115)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:92)
at
com.caucho.server.dispatch.ServletFilterChain.doFilter(ServletFilterChai
n.java:99)
at
net.exchangesolutions.sncore.servlet.StateFilter.doFilter(StateFilter.ja
va:56)
at
com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.
java:70)
at
com.caucho.server.webapp.WebAppFilterChain.doFilter(WebAppFilterChain.ja
va:163)
at
com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.j
ava:208)
at
com.caucho.server.hmux.HmuxRequest.handleRequest(HmuxRequest.java:396)
at
com.caucho.server.port.TcpConnection.run(TcpConnection.java:363)
at com.caucho.util.ThreadPool.runTasks(ThreadPool.java:490)
at com.caucho.util.ThreadPool.run(ThreadPool.java:423)
at java.lang.Thread.run(Thread.java:595)
Caused by: org.apache.tapestry.enhance.CodeGenerationException
at
org.apache.tapestry.enhance.javassist.ClassFabricator.getByteCode(ClassF
abricator.java:291)
at
org.apache.tapestry.enhance.javassist.EnhancedClass.createEnhancedSubcla
ss(EnhancedClass.java:130)
at
org.apache.tapestry.enhance.ComponentClassFactory.createEnhancedSubclass
(ComponentClassFactory.java:336)
at
org.apache.tapestry.enhance.DefaultComponentClassEnhancer.constructCompo
nentClass(DefaultComponentClassEnhancer.java:143)
... 50 more
Caused by: javassist.CannotCompileException: duplicate method: <init>
at
javassist.bytecode.ClassFile.testExistingMethod(ClassFile.java:488)
at javassist.bytecode.ClassFile.addMethod(ClassFile.java:472)
at javassist.CtClassType.addConstructor(CtClassType.java:885)
at javassist.CtNewClass.addConstructor(CtNewClass.java:55)
at
javassist.CtNewClass.inheritAllConstructors(CtNewClass.java:96)
at javassist.CtNewClass.toBytecode(CtNewClass.java:63)
at javassist.CtClass.toBytecode(CtClass.java:1021)
at
org.apache.tapestry.enhance.javassist.ClassFabricator.getByteCode(ClassF
abricator.java:279)
... 53 more

Michael Prescott
direct: 416.646.7062

main: 416.646.7000
fax: [phone]

Exchange Solutions Inc.
250 Yonge Street, 18th Floor
Toronto, ON M5B 2L7
www.exchangesolutions.com